Clara Ismeria Damiani Bica

A d.s.c. kinetic study on the effect of filler concentration on crosslinking of diglycidylether of bisphenol-A with 4,4′-diaminodiphenylmethane

Abstract The kinetics of formation of epoxy resin composites was studied using an epoxy matrix of diglycidylether of bisphenol-A (DGEBA) and 4,4′-diaminodiphenylmethane (DDM) as curing agent in the presence of quartz flour as filler. The curing reactions of these systems were kinetically followed by differential scanning calorimetry (d.s.c.) at different scanning rates. The activation energies of the curing reactions were obtained using non-isothermal methods: the single scanning rate method of Barrett as well as the multiple scanning rate methods of Kissinger, Osawa and half-width. The crosslinking process is characterized by an average activation energy of 54.2–62.7 kJ mol −1 despite different methods or filler concentrations being used to obtain it. The reaction rate for the systems with filler concentrations higher than 10 wt% showed a considerable decreasing at higher temperatures at conversions higher than 50%.

Cellulose and Nanocellulose from Maize Straw: An Insight on the Crystal Properties

In this work cellulose was extracted from corn/maize straw (Zea mays) by means of an environmental-friendly multistep procedure involving alkaline treatment and a totally chlorine-free bleaching. This multistep procedure efficiently removed lignin and hemicelluloses. The pulp resulting from each step was characterized by attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR). The optimum pulping time (time of alkaline treatment) was determined by means of thermogravimetric analysis. The extracted cellulose is highly crystalline as verified by X-ray diffraction. The partial acid hydrolysis with sulfuric acid led to the isolation of cellulose whiskers in aqueous suspension as confirmed by light scattering and transmission electron microscopy. The depolarization ratio value of these nanocrystals is the same as that determined for cotton whiskers, showing that this ratio does not depend on the cellulose source. The maize whiskers are arranged laterally in bundles with average thickness around five times that of the crystallite.

Physicochemical Properties of Methylcellulose and Dodecyltrimethylammonium Bromide in Aqueous Medium

Interactions between uncharged polymers and cationic surfactants are considered weaker than interactions with the anionic analogues. This work describes the binding occurring between methylcellulose (MC) and the cationic surfactant DTAB in aqueous medium. In the absence of salt, MC-DTAB exhibits a maximum in hydrodynamic radius, R(h,slow), with the increase in the surfactant concentration. Otherwise, in presence of salt the MC-DTAB system shows only a linear increase of R(h,slow). CAC is lower than the CMC, which is taken as an evidence of binding between the cationic surfactant and neutral polymer that induces the aggregation process. Static light scattering, rheology and micro-DSC results highlight the hydrophobic MC-DTAB association. Salt-out and the salt-in effects were observed in presence of DTAB, with a clear transition at concentration values close to the CMC, as judged from rheological and micro DSC measurements. Indeed, DTAB affects both the pattern of the sol-gel transition and the gel strength.

Role of the co-surfactant nature in soybean w/o microemulsions.

The influence of the co-surfactant on physicochemical properties of w/o soybean oil microemulsions (MEs) has been studied. In spite of the similarity in phase diagrams, the MEs display remarkable differences when examined by electrical conductivity, dynamic light scattering (DLS), small angle X-ray scattering (SAXS) and linear voltammetry. When different short-chain alcohols were employed as co-surfactants, together with sodium dodecyl sulfate (SDS) as surfactant, the DLS results indicated the systems to be monodisperse. Both the electrical conductivity of the MEs and the hydrodynamic radii of the droplets (R(H)) increased with water content while R(H) diminished as temperature increased, no aggregation or percolation of the droplets being observed. In comparison to w/o MEs prepared with 3-methyl-1-butanol, those prepared with 1-pentanol presented higher electrical conductivity and larger limiting currents at a Pt ultramicroelectrode for oxidation of the water occluded into the particles. Finally, from the electrochemical viewpoint the use of 1-pentanol is recommended, no advantage being gained by using any of the other tested alcohols.

Intermolecular Interactions of Polymer/Surfactants Mixture in Aqueous Solution Investigated by Various Techniques

The interaction between diblock copolymer with ionic surfactants in aqueous solution were studied employing surface tensiometery, electrical conductivity, steady-state fluorescence spectroscopy and dynamic laser light scattering at 303 K. Surface tension measurements were used to determine critical micelle concentration (CMC) and, thereby, its free energy of micellization (ΔGm), free energy of adsorption (ΔGads), surface excess concentration (Γ), and minimum area per molecule (A). The negative values of (ΔGm) both in case of SDS and CTAB confirmed the spontaneity of the processes. Conductivity measurements were used to determine critical micelle concentration (CMC), critical aggregation concentration (CAC), polymer saturation point (PSP), degree of ionization (α), and counterion binding (β). Addition of polymers could effectively reduce the CMC value of surfactants and, thus increase the detergency. Dynamic light scattering experiments were performed to check the changes in physiochemical properties of the block copolymer micelles take place due to the interactions of diblock copolymer with ionic surfactants. The ratio of (I1/I3) indicates the polarity of the pyrene micro environment and confirms the detection of micelle as well as polymer-surfactant interactions. Aggregation number (N), number of binding sites (n), and free energy of binding (ΔGb) for pure surfactants as well as for polymer-surfactant mixed micellar systems were determined by fluorescence quenching method.

Water in soybean oil microemulsions as medium for electrochemical measurements

Microemulsions of water in soybean oil (w/o ME) were prepared with sodium dodecyl sulfate (SDS) as surfactant and amyl or isoamyl alcohol, as co-surfactants. Microemulsions containing 40.0% oil, 43.2% alcohol, 10.8% SDS and 6.0% water in weight, in the ratio 1:4 [SDS]:[alcohol] showed the highest thermodynamic stability. The aqueous droplet size and its diffusion coefficient Dw/o in the ME were determined through dynamic light scattering (DLS). Voltammetric measurements in the ME at a Pt disk ultramicroelectrode (ume) evidenced the oxidation of both water and ferrocene (Fc), and the reduction of oleic acid. The Dw/o values calculated from the limiting current being lower than the ones obtained from DLS indicate that water oxidation probably requires diffusion towards the electrode of both the droplets and the water molecules from inside the droplets. The results show that electroanalytical determinations can be carried out in w/o ME.

Cellulose nanocrystals from acacia bark–Influence of solvent extraction

The isolation of cellulose nanocrystals from different lignocellulosic materials has shown increased interest in academic and technological research. These materials have excellent mechanical properties and can be used as nanofillers for polymer composites as well as transparent films for various applications. In this work, cellulose isolation was performed following an environmental friendly procedure without chlorine. Cellulose nanocrystals were isolated from the exhausted acacia bark (after the industrial process of extracting tannin) with the objective of evaluating the effect of the solvent extraction steps on the characteristics of cellulose and cellulose nanocrystals. It was also assessed the effect of acid hydrolysis time on the thermal stability, morphology and size of the nanocrystals, through TGA, TEM and light scattering analyses. It was concluded that the extraction step with solvents was important in the isolation of cellulose, but irrelevant in the isolation of cellulose nanocrystals. Light scattering experiments indicated that 30min of hydrolysis was long enough for the isolation of cellulose nanocrystals.

Anionic surfactant aggregation with (Hydroxypropyl)cellulose in the presence of added salt

-1 ) was studied. The surfactants were sodium cholate (CS), sodium deoxycholate (DC) and sodium dodecylsulphate (SDS). By fluorescence probing, the critical aggregate concentration (C 1 ) decreased for SDS and DC in comparison to the critical micelle concentration (CMC) whereas it increased for CS at 298 K. The relative viscosity reached a maximum for SDS/HPC but remained constant for CS/HPC and DC/HPC. By light scattering, cloud points were verified to increase. By dynamic light scattering, it was concluded that, at low surfactant contents, the fast mode of HPC is related to surfactant/HPC aggregates and shorter HPC chains; at high contents, to free micelles. The slow mode is linked to interchain polymer-surfactant complexes and HPC clusters. For the bile salts/ HPC systems, the mechanism of aggregation may occur in two steps.

Application of the half-width kinetic method on the amine-initiated cross-linking of an epoxy resin with cyclic anhydrides

Abstract The utility of a non-isothermal kinetic method—the half-width method—is demonstrated in the determination of the activation energy for amine-initiated cross-linking reactions of the epoxy resin diglycidylether of bisphenol-A with three different cyclic anhydrides: succinic, maleic and cis -1,2-cyclohexanedicarboxylic anhydrides. Triethylamine was employed as an initiator. The cross-linking reactions were kinetically followed by differential scanning calorimetry at various temperature scanning rates. The half-width method furnished values for the energy of activation in the range 60–100 kJ mol −1 . These values are similar to those obtained by applying the well-known non-isothermal multiple-scanning rate methods of Kissinger and Osawa but differ significantly from the ones obtained by applying the single-rate method of Barret.

Physical chemical properties and kinetics of redox processes in water/soybean oil microemulsions

Microemulsoes de agua em oleo (ME w/o) constituidas de agua, oleo de soja, SDS (dodecil sulfato de sodio) e diversos alcoois de cadeia curta foram caracterizadas sob o ponto de vista fisico-quimico e de propriedades eletroanaliticas. Foram testados diferentes co-surfactantes e razoes surfactante:cosurfactante, sendo a ME com a composicao mais favoravel utilizada para estudar a cinetica de processos redox. Para tanto, realizaram-se medidas de voltametria ciclica utilizando como eletrodo de trabalho um ultramicroeletrodo de disco de Pt e, como eletrodo de referencia e auxiliar, foram usados Ag/AgCl e Pt, respectivamente, e ferroceno como uma sonda. Verificou-se que a estabilidade termodinâmica das MEs aumenta com o conteudo de co-surfactante. A estrutura molecular e a solubilidade em agua do co-surfactante afetam a condutividade eletrica e o raio hidrodinâmico das MEs. Devido aos baixos coeficientes de difusao observados na ME, foi possivel a realizacao de medidas em estado transiente usando velocidades de varredura convencionais. A oxidacao do ferroceno na ME demonstrou ocorrer em condicoes de quasi-reversibilidade. Assim, foi demonstrada a possibilidade de realizar estudos por voltametria ciclica em oleos vegetais sob a forma de ME w/o. Water/oil microemulsions (w/o ME) constituted by water, soybean oil, SDS (sodium dodecyl sulfate) and several short-chain alcohols were characterized from the viewpoint of its physical-chemical and electroanalytical properties. Different co-surfactants and surfactant:co-surfactant ratios were used, and the ME with the most favourable composition was used to study the kinetics of redox processes. For this purpose, cyclic voltammetry measurements using a Pt disk working ultramicroelectrode, an Ag/AgCl reference electrode and a Pt auxiliary electrode, and ferrocene as a probe, were performed. It was verified that the thermodynamic stability of the MEs increases with the co-surfactant content. The molecular structure and water solubility of the co-surfactant affect the electrical conductivity and the hydrodynamic radio of the MEs. Owing to the low diffusion coefficients verified in the MEs, measurements in transient state using conventional sweeping rates could be performed. Ferrocene oxidation in the ME has been demonstrated to proceed in quasi-reversibility conditions. Thus, the possibility of carrying out studies of cyclic voltammetry in vegetable oils under the w/o ME form was demonstrated.